The present invention relates to a process of recovering monomeric esters of substituted or unsubstituted acrylic acids from polymer material having corresponding structural units.
Acrylate polymers, which include acrylic glasses chiefly consisting of polymethyl methacrylate (PMMA), are used for instance for the production of long-lived consumer goods. For this purpose, there are frequently used molding processes in the course of which considerable amounts of waste polymer may be obtained. For expediently reprocessing these production wastes and for utilizing waste materials recirculated from the process of utilization quite a number of proposals have now been made.
It is a well-known fact that acrylate polymers, above all PMMA, belong to the few plastic materials which are excellently suited for direct chemical recycling. This means that at certain temperatures and pressures these polymers can completely be decomposed again into the corresponding monomer units (depolymerization) when heat is supplied in an appropriate way (Grassie, N., Melville, H. W., Bull. Soc. Chim. Belges 1948, p. 142).
In the reports on the xe2x80x9c19. Kunststofftechnisches Kolloquium des Eurogress Aachenxe2x80x9d (Mar. 11-13, 1998) there is described a continuously operating process of depolymerizing PMMA. The comminuted plastic material is charged into a hot extruder (ZSK 30), in which two tightly meshing screws are rotating with a self-cleaning effect. By means of these screws, nondepolymerized PMMA and other residues are discharged from the extruder. The PMMA will depolymerize in the extruder due to the thermal and the mechanical shearing effect. The resulting MMA is withdrawn as vapor phase through the degassing bell and is condensed. In this process, the MMA content of the condensate varies between 89% and 97%, the yield of MMA is about  less than 97%. In the above-described process, heating the PMMA is effected in the extruder via the shell walls. The ratio of wall surface to reactor volume deteriorates, however, with increasing plant. For large plants on an industrial scale the available shell surface is so small that the extruder must either be made extremely hot, in order to sufficiently decompose the PMMA, or only very much worse yields of MMA are obtained. The necessary increased heating of the extruder shell, however, leads to a local overheating, which contributes to the formation of byproducts and impairs the monomer purity.
Furthermore, it is known to depolymerize PMMA by means of a fluidized-bed pyrolysis. As fluidized material there is used quartz sand having a grain size of 0.3 to 0.7 mm. It is a disadvantage of this process that in the course of time the fluidized material is graphitized with soot. When the soot chips off the sand grain, it can be entrained with the gas stream. To obtain an appealingly clean monomer, this plant therefore requires the implementation of many special filter systems (cooler, cyclone, electrostatic precipitator). In this method, a nitrogen stream is used for fluidizing the sand. It is likewise disadvantageous that after the depolymerization the nitrogen and the MMA gas must again be separated by cooling. The nitrogen stream, which upon separation from the product gas is recirculated to the reactor, must therefore be cooled and heated in constant alternation, where the temperature difference is at least 400 K. For a large-scale process, this is disadvantageous from an economical and ecological point of view (J. Franck, thesis 1993,Hamburg University).
It is the object of the invention to provide a process of recovering monomeric esters of substituted or unsubstituted acrylic acids from polymer material having corresponding structural units, which allows a continuous depolymerization free from residues, and thus provides for the production of high-quality recycled monomeric esters in a high yield. In accordance with the invention, free from residues is understood to be a process which avoids the formation of deposits in the reactor space and thus makes it superfluous to shut down the plant for removing the deposits, so that a continuous operation is ensured.
Furthermore, it is the object of the invention to provide a process as mentioned above, which can be operated on an industrial scale and helps to eliminate the disadvantages such as a poor transfer of heat during the depolymerization, a high amount of apparatus required as well as energetically unfavorable process flows.
The subject-matter of the invention is a process of depolymerizing PMMA, which is characterized in that in a reactor the polymer material is brought in contact with a hot mechanically fluidized solid (heat-transfer medium), and the resulting vapors are withdrawn and condensed, where the hot heat-transfer medium is continuously supplied at one end of the reactor, and cooled heat-transfer medium is discharged at the other end.
By means of the inventive process the reactor volume can be kept small as a result of the very good heat transfer of the fine-grained solid and the related relatively short dwell time of the polymer material. Hence, the dwell time of the resulting monomer vapors in the reactor is less than 6 seconds. The desired esters of the acrylic acids are obtained in very good yields and with a high purity. Thus, the hot, fine-grained heat-transfer medium also ensures that in large-scale plants a sufficient transfer of heat is ensured during the depolymerization.
In accordance with the invention, the mechanically fluidized fine-grained heat-transfer medium produces a rubbing effect in the reactor, which helps to completely prevent an accretion of byproducts resulting from the depolymerization at the inner walls and installations of the reactor. These depolymerization byproducts are continuously discharged from the reactor together with the fine-grained heat-transfer medium, so that an agglomeration of the byproducts in the reactor is prevented. Thus, it is possible to continuously perform the advantageous process, as no depolymerization residues, which otherwise must be removed from time to time from corresponding plants of the prior art, are left in the reactor. The monomer gas stream, which leaves the reactor, has a sufficient purity and need only be liberated from entrained dust particles by means of a cyclone. A separation from a carrier gas stream, which would even entrain more dust particles, is by no means necessary.
The mechanical fluidization and the transport of the fine-grained heat-transfer medium can be achieved by all possibilities well-known to the man skilled in the art, such as by moving or rotating walls, possibly under the influence of gravitation. There is preferred the embodiment where the substances supplied to the reactor are mechanically fluidized, mixed with each other and conveyed in a mixer by means of one or more intermeshing shafts rotating in the same direction, which are provided with coils or other mixing tools. Approximately the same dwell time of all solid particles (plug flow) in the range from 5 to 60 seconds can be adjusted by changing the rotational speed of the screws.
The polymer material is heated within a short period and depolymerized by means of the hot heat-transfer medium fluidized by the coils or mixing tools. The volatile components are discharged, whereas the solid byproducts remaining after the depolymerization are discharged from the reactor together with the heat-transfer medium, so that a contamination of the withdrawn monomer vapors with components originating from the solid byproducts is likewise very advantageously prevented. The mass balance of the heat-transfer medium in the reactor is preferably maintained by supplementing at the top end from a heated receiver.
As indicated above, the transport of the heat-transfer medium in the reactor can preferably be effected by one or more rotating shafts, which are equipped with coils or other mixing tools, from the inlet opening to the outlet opening.
Having been discharged from the reactor, the heat-transfer medium can be supplied to the bottom end of a pneumatic conveying line via a secondary degassing tank. The subsequent heating of the heat-transfer medium can basically be effected with all methods known to the man skilled in the art. Preferably, a hot, possibly oxygenous gas stream and/or possibly additional fuels are supplied to the bottom end of the pneumatic conveying line via the combustion chamber. The resulting gas stream supplies the heat-transfer medium to the top, where at the same time combustible residues from the depolymerization and additional fuels are burnt. Accordingly, this leads to the reheating of the fine-grained heat-transfer medium. The mixture of heat-transfer medium and gases reaches a heat transfer separator, from which gases and fine dust particles (e.g. color pigments contained in the PMMA) are withdrawn via a dust separator (cyclone, exhaust gas filter). The heat-transfer medium separated in the heat transfer separator trickles downwards and reaches a collecting tank which serves as hot heat transfer receiver for the reactor.
The temperature of the heat transfer solid at the reactor inlet depends on the ratio of the mass flows heat-transfer medium/polymer material. With a ratio of 10:1 a superheating of the heat-transfer medium of 150xc2x0 C. is obtained, with a ratio of 5:1 a superheating of 300xc2x0 C. During the depolymerization, the reaction mixing temperature of the heat-transfer medium can lie in the range between 300xc2x0 C. and 650xc2x0 C., preferably between 350xc2x0 C. and 450xc2x0 C. However, the heat-transfer medium heated by the hot gas stream has a temperature of 400xc2x0 C. to 900xc2x0 C., preferably 500xc2x0 C. to 750xc2x0 C.
As heat-transfer medium, every inorganic fine-grained solid (grain size between 0.1 and 5 mm, preferably 0.3 and 2.0 mm) can be used, which has the required strength and a sufficient stability with respect to temperature changes and oxygen. In many cases, screened sand was useful, which according to DIN 4222 is called coarse sand. However, there may also be used other naturally occurring or synthesized oxides on the basis of silicon, aluminium, magnesium, zirconium or also mixtures of these elements.
With the process in accordance with the invention, the dwell time of the vapors and gases formed in the reactor before the condenser can be less than 6 seconds, preferably less than 2 seconds. The dwell time of the fine-grained heat-transfer medium in the reactor is freely selectable. It is preferably in the range from 5 to 60 seconds.
The ratio of hot heat-transfer medium to PMMA in the reactor is likewise freely selectable in wide ranges. What seems to be expedient and preferred is a ratio between 3:1 and 30:1.
In accordance with a further aspect, the inventive process can improve the economy of the recovery as well as the quality of the product obtained in that the vapors withdrawn are treated with condensate, which was cooled in a monomer circulating cooler, and in that the condensate resulting from the treatment is recirculated to the cooler, where it is cooled and partly recirculated for treating the vapors withdrawn, and the remaining part of the condensate is discharged for further processing and product recovery.
The crude vaporous depolymerizate, i.e. the depolymerization gases, is first of all sprayed by means of a nozzle as in a shower with a part of the condensate previously cooled in the monomer circulating cooler and recirculated. Due to the direct contact of the depolymerization gases with the atomized crude condensate liquid a fast intensive cooling and a very short dwell time of the vaporous depolymerizate at the depolymerization temperature is achieved, which leads to a distinct improvement of the monomer yield and quality. The vapors withdrawn are quenched with condensate in a concurrent process. Solid deposits, which are produced on the otherwise usual coolers, can thereby be reduced considerably. Since the actual coolers only get in contact with a distinctly cooler, already condensed crude product, the risk of accretion or deposits is reduced. A reduction of the coating thickness on coolers can, however, additionally contribute to an improved quality of the crude condensate, in particular to an increased monomer content.
The condensate used for treating the monomer gases can be cooled in the monomer circulating cooler to a temperature between about 5 and 40xc2x0 C., preferably 20 and 30xc2x0 C. With parts of this cooled condensate, the vapors withdrawn from the reactor are then expediently cooled during quenching to a condensate temperature between 20 and 50xc2x0 C., preferably 35 and 50xc2x0 C.
The non-condensable vapors and gases of the condensate are expediently introduced into the riser, in which the fine-grained heat-transfer medium is heated, and also burnt.
In principle, the polymer material can be supplied to the reactor in every conceivable form. The commonly used introduction devices were successfully used in conjunction with conveyor belts, screws or the like. Larger pieces such as plates or molded articles can easily be comminuted to the size required or desired for the reactor, for instance by preceding shredders or mills. In the final analysis, the size of the processable polymer material pieces also depends on the properties of the polymerizates and the capacity of the reactor.
In general, the polymer material to be fed into the reactor and to be depolymerized there may be present in any conveniently processable form, for instance as chips, as granules, as fine powder, as shavings or as coarsely shredded material. These forms of addition may be introduced alone or in combination. Moreover, the more or less solid forms of addition can also be introduced together with liquid monomer in a more or less pure or contaminated form. When performing the invention, granules with a preferred grain size of about 1 to 10 mm were particularly useful.
The polymers to be fed into the reactor in the process in accordance with the invention chiefly contain structural units which in terms of their chemical structure satisfy the following formula I: 
wherein
R1 is C1-6alkyl, preferably C1-4alkyl,
R2 is H, C1-6alkyl, preferably H or C1-4alkyl, with H or CH3 being particularly preferred, and
n is a positive integer larger than 1.
Exemplary compounds include polymethyl acrylate, polyethyl acrylate, polymethyl methacrylate, polypropyl acrylate, polybutyl acrylate, polypropyl methacrylate, polybutyl methacrylate and copolymers which have two or more of these types of polymer. The first four compounds are preferred within the scope of the invention. Particularly preferred is polymethyl methacrylate (PMMA).
In addition to the chemical mixtures (random copolymers or also block copolymers), which were obtained by copolymerizing at least two substituted or unsubstituted acrylic-acid ester monomers (e.g. methylmethacrylate-n-butylmethacrylate copolymers), copolymers can be processed with the inventive process which have up to 50 wt-% of at least one further vinylically unsaturated monomer which can be copolymerized with at least one substituted or unsubstituted acrylic-acid ester monomer.
Typical examples include for instance methylmethacrylate-styrene copolymers or methylmethacrylate-butylacrylate-styrene terpolymers.
Physical mixtures, so-called blends, can also be reprocessed in accordance with the invention. As regards the reprocessing of polymer material in accordance with the inventive process, merely the fundamental possibility of the nondestructive depolymerization (with respect to the monomers) as well as the possibility of separating the resulting vapor mixture in a fractionated distillation or with other commonly used methods of separation must be seen as limiting factors. When depolymerization and separation are possible in principle, there is no fundamental obstacle to using the process in accordance with the invention.